The present invention relates to a method for evaluating semiconductor wafers and to a device for use therein. More specifically, the present invention relates both to a method and its device for estimating the dopant level in semiconductor wafers and to a method and its device for measuring the lifetime in semiconductor wafers.
The recent trend toward ultrahigh levels of accuracy in semiconductor devices, as typified by ultralarge-scale integration, requires that the semiconductor wafers used for this purpose be subjected to an ever more rigorous quality control.
In achieving this control, it is highly desirable that semiconductor wafer evaluation be carried out by a noncontact measurement method that does not risk contaminating or damaging the semiconductor wafer. An example of such a method is the use of microwaves to measure semiconductor properties.
FIG. 1 contains a block diagram of the measurement principle for one example of the prior art devices for measuring semiconductor properties. FIG. 2 contains an input/output time chart of this device for measuring semiconductor properties. FIG. 3 contains a graph of the relationship between semiconductor wafer resistivity and the variation in the value of the microwave detection current. FIG. 4 contains an attenuation curve as displayed on a synchroscope.
As demonstrated on FIG. 1, the prior device A for measurement of semiconductor properties is constituted of the following: a sample support 1, an excitation light generator 4 that generates excitation light that is emitted onto a semiconductor wafer 2 (measurement sample) held on sample support 1, a microwave generator 5 that generates microwaves that are emitted onto semiconductor wafer 2, a waveguide 3 that conducts the microwaves from microwave generator 5 onto semiconductor wafer 2, a detector 6 that detects (via waveguide 3) the microwaves reflected from the surface of the semiconductor wafer 2, a synchroscope 7 that displays the microwave level variation as detected by the detector 6, and a control circuit 4 that controls excitation light generator 4, microwave generator 5, detector 6, and synchroscope 7.
The corresponding measurement principle is explained below with reference to FIG. 2.
A known microwave level b generated by the microwave generator 5 is emitted from the emitter tip of waveguide 3 onto the surface of semiconductor wafer 2. A light pulse a, of known level, width, and interval, is generated by excitation light generator 4 and is emitted as excitation light onto the surface of the semiconductor wafer 2 that is also receiving microwave radiation b, thereby exciting the carriers (free electron and positive hole pairs) within the semiconductor wafer 2. Since the majority carrier is originally present in the semiconductor wafer 2 in very large numbers, it undergoes only a relatively small change in concentration due to exposure to this excitation light. However, the minority carrier undergoes a substantial change in concentration, and the properties of the semiconductor wafer 2 can be very accurately detected by detecting the change in the minority carrier concentration. Thus, the discussion that follows will generally focus on the minority carrier.
Because the minority carriers excited as above represent an excess over the carrier concentration when semiconductor wafer 2 is at thermal equilibrium, the minority carrier concentration d increases.
Then, between pulses (interruption in photoexposure), these excess carriers recombine and their population gradually declines. As a result, the minority carrier concentration d also gradually declines until it reaches the level at thermal equilibrium.
The exponential function exp(-t/.pi.) governs the time required for restoration of the thermal equilibrium condition after this type of displacement of the minority carrier from thermal equilibrium (t=time elapsed from cessation of photoexposure). The parameter T is called the minority carrier lifetime, and it is one of the parameters that can represent the impurity concentration in a semiconductor wafer.
Due to the change in electrical conductivity (i.e., resistivity) in semiconductor wafer 2 that accompanies the change in minority carrier concentration d, the relationship depicted in FIG. 4 obtains between the resistivity and the detection current value. This detection current value corresponds to changes in the level of the microwave radiation that has been emitted onto the semiconductor wafer 2. After the level change, the reflected microwave radiation c (reflected by the surface of the semiconductor wafer 2) again passes through waveguide 3 and travels to detector 6. The microwave (reflected wave) attenuation detected by synchroscope 7 is expressed by an attenuation curve, for example, as in FIG. 4.
The level of impurities such as, for example, heavy metals, in semiconductor wafer 2 can be evaluated from measurement of the minority carrier lifetime based on this attenuation curve.
In the above-described microwave-based method for measuring semiconductor properties, the detection accuracy for the change in microwave level is reduced when the carrier injection density (corresponds to the exposure) is reduced at the point of production of the excess minority carriers by exposing semiconductor wafer 2 to the excitation light.
In other words, a diminution in the carrier injection density also diminishes the magnitude of the variation in minority carrier concentration between the thermal equilibrium and injection states. Due to this, the level change in the reflected microwaves becomes smaller, the S/N ratio is diminished, and the detection accuracy is reduced.
For this reason, based on considerations of the detection accuracy of the device A for measuring semiconductor properties, the carrier injection density in prior measurement methods has been established taking into account a safety factor with respect to the detection limit.
The semiconductor characteristics are then evaluated by measuring the minority carrier lifetime while fixed at the carrier injection density established on the preceding basis.
However, when the carrier injection density is established at a high injection state at or above some specified value, the injected carrier density can become dominant or controlling, with the result that the density values for the majority carrier and minority carrier within semiconductor wafer 2 become approximately equal. This generates a carrier migration phenomenon known as bipolar diffusion. Bipolar diffusion substantially affects minority carrier lifetime. In addition to this bipolar diffusion, the following nonlinear phenomena are produced at high injection states: variations in the diffusion constant (indicates the relationship between carrier migration distance {diffusion path} and carrier lifetime), Auger recombination in which recombination of electron-positive hole pairs occurs by a phenomenon other than the aforementioned photoexcitation recombination, and saturation of the occupied states of the recombination centers by excess carriers. Due to the complex interactions among these phenomena, the risk arises that the measured minority carrier lifetime will differ from the native minority carrier lifetime as derived from the known theoretical equation according to Shockley-Read-Hall statistics [W. Shockley and W. T. Read, Phys. Rev., 87, 835 (1952); R. N. Hall, Phys. Rev., 87, 387 (1952)]. As a result, the semiconductor wafer 2 may not be accurately evaluated in some cases.
Furthermore, the change in the microwave level extends into the nonlinear segment of the microwave detection curve at high injection states, and this creates additional risk of error in measurement of the minority carrier lifetime.